In the state of the art, it is known to measure a temperature via a resistance element, whose electrical resistance depends on the temperature existing in its surroundings. The resistance element forms, thus, the temperature sensor of a resistance thermometer. In order, in the case of such measuring devices, to determine, sufficiently exactly, the resistance of the temperature sensor and, therewith, temperature, a hookup of the sensor with at least three electric lines/connections is necessary. In the case of such a 3-wire circuit, it is assumed that the electrical resistances of all 3 lines are equal, i.e. equal line lengths and equal temperature of the connection wires is assumed. By measuring the resistance of the lines located on the same side of the resistance element, the line resistance can be determined. This line resistance is then subtracted from the measured total resistance through the lines lying opposite one another on the resistance element, in order to determine the resistance of the sensor itself. This continual measurement of the electrical resistance of the connecting lines is important, since the resistance value of the lines is likewise temperature dependent.
The electrical connecting lines are usually connected to a measurement transmitter, which operates the temperature sensor suitably and evaluates or conditions the measured values, or transmits the measured values to a display unit or via a databus to a superordinated unit. Such a temperature head transmitter for installation in a connection head of the Form B according to DIN 43729 has usually 6 connection terminals: One terminal each for the current supply and the output signal produced by the transmitter and four terminals for connection of the sensor. There are, however, some applications, in which two sensors are connected, e.g. two thermocouples or two resistance thermometers. For the measuring of two resistance sensors (e.g. resistance thermometers or resistance-dependent position transducers) in, in each case, a 3-line circuit, at least 5 terminals are required. One terminal is, in such case, connected simultaneously with two connecting wires. The placement of a fifth sensor terminal on a DIN B head transmitter is very difficult, since, on the one hand, the terminals become too small for manual working, or less space remains for a parametering interface and lateral cable lead-in for power supply/signal. The problem increases in the case of head transmitters with 4 terminals, in which case, the only possibility offered is for simultaneous connection of one 3- and one 2-wire resistance sensor. This leads to extreme inaccuracies in the case of the 2-line resistance sensor, since, for this, no information is present concerning the temperature-dependent resistance of the connecting lines.